How can I get a stereo cameras volume and parameters?
The code in properties_data_camera.py can give you clues to most of the camera properties.

The volume options for the 3DView are displayed with code in space_view3d.py and use properties within the 3DView data eg. context.space_data.show_stereo_3d_volume.

The volume pyramid drawn for this option is based on a pyramid created from the camera location/rotation, using cam.lens which is shown to the user as focal length and cam.sensor_width (sensor size). The cam.clip_start and cam.clip_end define where the top and bottom of this pyramid are. The actual c-code that does the drawing can be found in drawobject.c

The location of the convergence plane is based on stereo.convergence_distance and looks to use the same calculations as clip_start with the outer edges based on the same volume pyramid.